#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=1e5+1000;
typedef long long LL;
inline LL read(){LL x=0,f=1;char ch=getchar();	while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
char s1[maxn],s2[maxn],s3[maxn];
int main(void)
{
  cin.tie(0);std::ios::sync_with_stdio(false);
  LL n,t;cin>>n>>t;
  cin>>(s1+1);cin>>(s2+1);
  for(LL i=1;i<=n;i++){
    for(LL j=0;j<26;j++){
        if(('a'+j)!=s1[i]&&('a'+j)!=s2[i]){
            s3[i]=('a'+j);
            break;
        }
    }
  }
  bool flag=1;
  LL cnt1=n-t;LL cnt2=n-t;
  for(LL i=1;i<=n;i++){
     if(cnt1>0&&cnt2>0&&s1[i]==s2[i]){
        cnt1--;cnt2--;
        s3[i]=s1[i];
     }
  }
  for(LL i=1;i<=n;i++){
     if(cnt1>0&&s3[i]!=s1[i]){
        cnt1--;
        s3[i]=s1[i];
     }
     else if(cnt2>0&&s3[i]!=s2[i]){
        cnt2--;
        s3[i]=s2[i];
     }
  }
  LL num1=0;LL num2=0;
  for(LL i=1;i<=n;i++){
    if(s3[i]!=s1[i]) num1++;
    if(s3[i]!=s2[i]) num2++;
  }
  if(num1!=t||num2!=t){
    cout<<"-1"<<"\n";
  }
  else{
    for(LL i=1;i<=n;i++){
        cout<<s3[i];
    }
    cout<<"\n";
  }
return 0;
}
